Method and plant for thickening sludge derived from water treatment by flocculation- decantation with ballasted floc

ABSTRACT

The invention concerns a method for thickening sludge derived from a water treatment plant by flocculation-decantation with floc ballasted with micro-sand or other dense granular material using a liquid cyclone treatment ( 1 ) of the decantation sludge. The invention is characterised in that it comprises successive steps which consist in: degassing ( 3 ) the overflow sludge of the liquid cyclone treatment ( 1 ) from the ballasted flocculation decantation unit, said degassing step being carried out on a corresponding degassing surface at a degassing surface velocity of less than 100 mph; injecting at least a flocculating reagent ( 4 ) into said sludge; and thickening said sludge in at least a laminar sludge thickener ( 6 ), with a stilling floor load calculated at the thickener floor surface higher than 200 kg of suspended matter per m 2  per day and the extracted sludge concentration higher than 10 g/l.

[0001] The invention relates to the field of treatment of sludge derivedfrom water treatment methods.

[0002] More precisely, the invention relates to the field of thetreatment of sludge derived from plants for treating water byflocculation-decantation with floc ballasted by micro-sand or by anequivalent material.

[0003] In water treatment plants, the sludge extracted from settlersseparating the substances able to be settled from the water to betreated may have been thickened at the bottom of the sludge settler, upto concentrations of dry matter exceeding 15 grams per litre, and ableto reach more than 100 grams per litre. These thickening settlers oftencomprise a recirculation loop for the sludge, recirculating part of thesludge extracted from the bottom of the settler to the flocculationzone.

[0004] This technique has the disadvantage of keeping the sludge in thebottom of the settler for a long time period, which may result in thephenomena of fermentation and of salting out pollution in the sludgesettler.

[0005] Therefore specific structures are often provided for thickeningthe sludge, to which the settled sludge is directed to thicken, andwhich alter the concentration of this sludge, which is often below 10 or5 grams per litre, to a concentration higher than 15 or even 50 gramsand up to more than 100 grams per litre.

[0006] These thickening structures, called thickeners hereinafter, arelike settling basins in which the sludge to be thickened is introduced,while the sludge thickened by the effect of gravity, often with the aidof a turning rake facilitating the separation of flocs during settlingfrom the water capturing them, is extracted from the bottom of thethickness. The water freed from the thickened sludge is recuperated byoverflow, in the upper part of the thickener.

[0007] These thickeners can take the form of single element settlers or,more rarely, plate settlers. Their dimensions are adjusted by thoseskilled in the art, for loads at the sill (division of the daily load ofmatter in suspension (MIS) to be thickened, expressed in kilograms ofmatter in suspension per day, by the surface area of the base of thethickener, usually called the sill surface) not exceeding 120kgMIS/m²/day, except in cases of very specific sludge such as sludgeafter carbonate removal, very charged in mineral particles. Thus theMemento Technique de L'Eau (ninth edition, volume 2 page 921) describesmass flows of from 80 to 120 kgMIS/m²/day for fresh primary sludge, offrom 45 to 70 kgMIS/m²/day for primary sludge+fresh activated sludge,and of from 15 to 25 kgMIS/m²/day for Drinking Water flocculation sludgewith metallic hydroxides (without silt)

[0008] In the case of physico-chemical settling flocculation ballastedwith granular matter, especially those described in patents FR 2627704and FR 2739094 by the Applicant, the settlers are calculated on the baseof very high settling speeds (division of the flow rate of treated waterin m³/hr by the surface area of “mirror” settling), able to reach up to50 to 100, or even 200 m/hr, when treating water with MIS concentrationable to reach 1, or even more than 2 gm MIS/litre: a simple calculationshows that such settling basins, producing per m² 100 kg/MIS/hr and perm² surface area of mirror settling when treating waters withconcentration 1 gm/l at 100 m/hr, will require a sill surface ofthickness of at least 24 m² per m² of mirror settling surface, when thethickness is dimensioned in the traditional way on the basis of 100kgMIS/m² sill/day maximum.

[0009] Such thickening surfaces are generally not very compatible withthe compactness sought when using rapid settling basins with ballastedfloc of micro-sand.

[0010] The aim of the present invention is to solve these problems.

[0011] This aim is achieved by means of the invention which relates to amethod for thickening sludge issued from at least one water treatmentunit by flocculation decantation with floc ballasted with micro-sand orother dense granular material, implementing a hydrocycloning stage ofthe settled sludge, characterised in that it comprises successive stagesconsisting of:

[0012] a degassing stage for the overflow sludge from the hydrocycloningof the ballasted flocculation decantation unit, said degassing stagebeing implemented over a degassing surface corresponding to asuperficial degassing speed of less than 100 m/hr.

[0013] an injection stage of at least one flocculation agent in saidsludge;

[0014] a thickening stage of said sludge in at least one lamellarthickener, with a load at the sill calculated on the sill surface of thethickener greater than 200 kgMIS/m²/day and the concentration ofextracted sludge greater than 10 gm/l.

[0015] In fact, the Applicant noted that the sludge issued from watertreatment by flocculation decantation with ballasted floc of granularballast, such as micro-sand has, as long as there is degassing at theoverflow outlet from the hydrocyclone separating it from the recycledballast material, and with prior reflocculation, an unexpected aptitudefor rapid sludge thickening.

[0016] According to an advantageous variant, the method comprises asupplementary stage for injection of at least one coagulating agentupstream of the injection of flocculating agent. Such an addition makesit possible to obtain, if needed, improvement in the quality of theoverflows.

[0017] Preferably, said degassing stage is implemented over a degassingsurface corresponding to a superficial degassing speed less than 60m/hr.

[0018] Preferably also, said injection stage of flocculating agent iscarried out within an agitated flocculation zone with a stay time of 10minutes or less, and preferably 4 to 6 minutes, the surface of theflocculation zone being able to constitute all or part of the degassingzone.

[0019] According to an advantageous embodiment of the invention, saidthickening stage is carried out in a lamellar thickener in which theunder-blade free height is comprised between 2 and 4 metres, the methodcomprising a control stage of the height of the sludge blanket enablingit to be maintained between two predefined heights, each of theseheights being at least 1.5 metres.

[0020] When said sludge is issued from tertiary sewage treatment, saidthickening stage is preferably carried out with a load at the sillgreater than 300 kg/m²/day for a concentration of extracted sludge ofmore than 15 gm/l.

[0021] When said sludge is issued from primary sewage treatment, oroverflows of combined sewage from rain, said thickening stage ispreferably carried out with a load at the sill greater than 1000kg/m²/day for a concentration of extracted sludge of more than 25 gm/l.

[0022] When the sludge to be thickened is issued from the settlingtreatment of surface water other than carbonate removal treatment, saidthickening stage is preferably carried out with a load at the sillgreater than 200 kg/m²/day for a concentration of extracted sludge ofmore than 10 gm/l.

[0023] In practice, one can thus use thickeners of from 2 to more than12.5 times more compact than traditional thickeners dimensioned at lessthan 120 kg/m²/day, without this requiring (but without excluding) thepresence of a recirculation loop for sludge between the bottom of thethickener and the flocculator.

[0024] Thus the invention makes it possible to thicken ballastedflocculation decantation sludge in very compact structures, up toconcentrations which can exceed:

[0025] 20 gm/l for tertiary treatment sludge by ballasted flocculationof sewage treated biologically prior to ballasted flocculation,

[0026] 35 gm/l for sludge from treatment by ballasted flocculation ofprimary sewage, or overflows from storm sewers,

[0027] from 10 to more than 100 gm/l for sludge issued from treatment byballasted flocculation of water destined for drinking water, beforefiltration.

[0028] The concentrations given above require the maintenance (in thecase of continuous or semi-continuous extraction of sludge from thebottom of the thickener), or the periodic raising (in the case ofperiodic extraction of sludge by opening the valve at the bottom of thethickener) of the sludge blanket up to a height of at least two metres.

[0029] It is to be noted that such loads at the sill impose, dependingon the concentration of sludge to thicken and the sludge thickened, highspeeds at the mirror (division of the overflow water flow rate of thethickener by the mirror surface area of the upper decantation zone).These high speeds at the mirror are accepted without abnormal losses ofmaterials in suspension (MIS) in the overflow waters due to theinterposition of decantation plates in the upper part of the thickener,in such a way as to maintain the Hazen speed (division of the overflowwater flow rate from the thickener by the total surface projected by theplates on the horizontal plane) within limits compatible with thequality of overflow required.

[0030] According to a variant of the invention, the overflow waters fromsaid thickening stage are returned to the head of said lamellardecantation plant with ballasted floc.

[0031] According to another variant of the invention, the method doesnot comprise a recirculation stage for sludge coming from saidthickening stage to said injection stage of flocculating agent.

[0032] According to a further variant, the method comprises arecirculation stage for sludge coming from said thickening stage to saidinjection stage of flocculating agent.

[0033] The invention also relates to any lamellar thickening plant forsludge issued from at least one water treatment unit by flocculationdecantation with floc ballasted with micro-sand or other dense granularmaterial, characterised in that it presents: at least one degassing zoneable to be produced in the form of one or several channels or tank witha minimum total surface area corresponding to a degassing speed of 100m/hr relative to the entering sludge flow rate, means for injection ofat least one flocculating agent and at least one thickening settlercomprising a lamellar decantation zone, a thickening zone for sludgeunder plates, an extraction device for sludge and an extraction zone forclarified overflow water above the plates, the surface at the sill ofsaid thickener being less than that obtained by dividing the flow of MISto thicken expressed in kg MIS/day for a minimum admissible load of 200kg/m²/day.

[0034] Preferably, the invention also comprises means for injection of acoagulating agent provided upstream from said injection means for theflocculating agent.

[0035] According to an advantageous variant of the invention, themaximum total surface of said degassing zone corresponds to a degassingspeed of 60 m/hr relative to the flow rate of entering sludge.

[0036] Advantageously, the plant according to the invention comprises atleast one agitated flocculation tank, whose surface can be the whole ofor part of the degassing zone.

[0037] Preferably, said thickener settler comprises plates of length ofbetween 0.5 metres and 3 metres, typically 1.5 metres, with a spacingcomprised between 5 and 15 cm, and preferably between 7.5 and 10 cm,with an angle formed relative to the horizontal greater than 55°, andpreferably 60°, and in which the free height between sill and bottom ofthe plates is comprised of between 2 and 4 metres, said thickenersettler comprising a sludge raking device and comprising sensors formonitoring the height of the sludge blanket thus making it possible tomaintain the latter between two pre-defined heights, each being of atleast 1.5 metres.

[0038] According to a variant, the plant according to the presentinvention has no recirculation loop for sludge between the lower part ofsaid thickening zone and said flocculation zone.

[0039] According to another variant, the plant has a recirculation loop,constituted of piping and a sludge pump, recirculating part of thesludge between the lower part of said under-blade thickening zone andsaid flocculation zone.

[0040] The invention together with its various advantages will be betterunderstood by reading the following description of a non-limitingembodiment mode of the invention provided with reference to FIG. 1,showing a plant such as this diagrammatically.

[0041] With reference to FIG. 1, a water treatment unit 2 forflocculation decantation with floc ballasted with micro-sand comprises,in the traditional way a coagulation zone 2 a, a flocculation zone 2 band a lamellar settling zone 2 c. The mixture of micro-sand and sludgeextracted from the lamellar settling zone is carried by channel 10 to ahydrocycloning unit 1 where the micro-sand is separated from theremainder of the sludge. The micro-sand thus separated is re-injectedinto the water treatment unit 2 by flocculation decantation.

[0042] According to the invention, the sludge exiting from thehydrocycloning unit 1 is carried by a channel 11 or is degassed in adegassing zone 3 before being carried to a thickener 6 with adecantation plate zone including inclined plates 6 a, a sludgethickening under-blade zone 6 b, an extraction device 7 for sludge andan extraction zone for clarified overflow water 9 above the blades 6 a.

[0043] This zone 3 can take on any form whatsoever making it possible toseparate the sludge from the gases it has caught during hydrocycloning,such as a specific tank, or all or part of the channel carrying thehydrocycloned sludge to the thickener 6. In the embodiment shown in FIG.1, this degassing zone 3 is in the form of a basin provided justupstream from the thickener 6.

[0044] The degassing zone 3 is dimensioned on the basis of a degassingsurface area greater than the ratio of the flow rate of sludge to bethickened divided by a superficial speed of less than 100 m/hr,preferably of less than 60 m/hr and even more preferably of less than 30m/hr.

[0045] After degassing, a polymer, generally anionic, is added to thesludge, at the level of from 0.5 to 5 gm/m³ of sludge to thicken, andtypically of from 1 to 3 gm/m³.

[0046] This polymer injection 4 can be carried out on line, beforeinjection in the thickener 6, but is preferably carried out, as shown inFIG. 1, in a slightly agitated flocculator 5, with a hydraulic stay timein the latter of the order of 10 minutes or less, and preferably of from4 to 6 minutes. As flocculating agent, a polymer can be used, forexample, at a level of 0.5 to 5 mg active material/litre of sludge tothicken.

[0047] The flocculated sludge is then introduced under the plates 6 a ofthe thickener 6, or in the plates by their edge.

[0048] The plates 6 a have a length comprised between 0.5 metres and 3metres, and typically 1.5 metres, and have a spacing comprised between 5and 15 cm, and preferably between 7.5 and 10 cm; their angle relative tothe horizontal is greater than 55° and preferably equal to 60°; theirsetting is envisaged to ensure that the overflows have a Hazen speedlower than 10 m/hr, and typically of from 1 to 3 m/hr.

[0049] The sludge thickens in the lower part of the thickener 6, asufficient free height (typically between 2 and 4 metres) being providedbetween the bottom of the plates and the sludge evacuation valve 7, towhich the thickened sludge is carried by gravity alone, in the case of apyramidal extraction hopper with steep slope, or by a scraper, in thecase of a bottom with a small slope. Preferably, a rake 8 is installedto improve thickening.

[0050] The sludge is extracted either by maintaining the level of thesludge blanket between two pre-defined heights, under the control of asludge level sensor, or by periodically opening the extraction valve,with or without monitoring the concentration of sludge extracted using aconcentration sensor.

[0051] The water separated from the thickened sludge is recuperated byoverflow 9 from the thickener 6, typically by spouts ensuring a goodhydraulic spread of water extraction.

[0052] According to its quality, the overflow water can be mixeddirectly with the water treated by the flocculator-decanter withballasted floc, or can be returned to the head of the treatment byballasted flocculation decantation, as shown in FIG. 1.

[0053] As example, a lamellar thickener produced according to the basisof the principle above, with a degassing zone of superficial speed 60m/hr, a pre-flocculation by direct injection of flocculent in the pipingfor sludge supply for the thickener under the plates, four plates at 600relative to the horizontal, of width 1 m and length 1.5 metres,separated by 75 cm, an under-plate thickening zone of height 3 metres,of cylindrical-pyramid shape, the thickener bottom pyramidal hopperhaving sides inclined at 60° relative to the horizontal, without rake inthe thickening zone, and with extraction under a height of 1.5 metresminimum, made it possible to obtain the following performance onballasted decantation sludge for primary municipal sewage:

[0054] concentration of sludge to thicken: 2 gmMIS/l

[0055] concentration of thickened sludge: >30 gmMIS/l

[0056] load at the sill: 1000 to 1500 kgMIS/m² sill/day

[0057] with concentration overflows: <300 mgMIS/l.

1. Method for thickening sludge derived from a water treatment unit by flocculation decantation with floc ballasted with micro-sand or other dense granular material, implementing a hydrocycloning stage of the settled sludge, characterised in that it comprises successive stages consisting of: a degassing stage for the overflow sludge from the hydrocycloning of the ballasted flocculation decantation unit, said degassing stage being implemented over a degassing surface corresponding to a superficial degassing speed of less than 100 m/hr. an injection stage of at least one flocculation agent in said sludge; a thickening stage of said sludge in at least one lamellar thickener, with a load at the sill calculated on the sill surface of the thickener greater than 200 kgMIS/m²/day and the concentration of extracted sludge greater than 10 gm/l.
 2. Thickening method according to claim 1, characterised in that it comprises a supplementary injection stage for at least one coagulating agent upstream from the injection of the flocculating agent.
 3. Thickening method according to one or the other of claims 1 or 2, characterised in that said degassing stage is implemented over a degassing surface corresponding to a superficial degassing speed lower than 60 m/hr.
 4. Thickening method according to any one of claims 1 to 3, characterised in that said injection stage of flocculating agent is carried out within an agitated flocculation zone with a stay time of 10 minutes or less, and preferably 4 to 6 minutes, the surface of the flocculation zone being able to constitute all or part of the degassing zone.
 5. Thickening method according to any one of claims 1 to 4, characterised in that said thickening stage is carried out in a lamellar thickener in which the under-blade free height is comprised between 2 and 4 metres, and in that it comprises a stage for monitoring the height of the sludge blanket making it possible to maintain the latter between-two pre-defined heights, each of these heights being at least 1.5 metres.
 6. Thickening method according to any one of claims 1 to 5, characterised in that said sludge is issued from tertiary sewage treatment, and in that said thickening stage is carried out with a load at the sill greater than 300 kg/m²/day for a concentration of extracted sludge greater than 15 gm/l.
 7. Thickening method according to any one of claims 1 to 5, wherein the sludge is issued from primary sewage treatment, or from combined sewage overflows at times of rain, and in that said thickening stage is carried out with a load at the sill greater than 1000 kg/m²/day for a concentration of extracted sludge of more than 25 gm/l.
 8. Thickening method according to any one of claims 1 to 5, wherein the sludge to be thickened is issued from the settling treatment of surface waters other than carbonate removal treatment, and in that said thickening stage is carried out with a load at the sill greater than 200 kg/m²/day for a concentration of extracted sludge of more than 10 gm/l.
 9. Thickening method according to any one of claims 1 to 8, wherein the overflow waters from said thickening stage are returned to the head of said lamellar decantation plant with ballasted floc.
 10. Thickening method according to any one of claims 1 to 9, wherein it does not comprise a stage for recirculating sludge from said thickening stage to said flocculating agent injection stage.
 11. Thickening method according to any one of claims 1 to 9, characterised in that it comprises a recirculation stage for sludge issued from said thickening stage to said flocculating agent injection stage.
 12. Lamellar thickening plant for sludge issued from at least one water treatment unit (2) by decantation flocculation with floc ballasted with micro-sand or other dense granular material, characterised in that it has at least one degassing zone (3) able to be produced in the form of one or several channels or basin with a total minimum surface corresponding to a degassing speed of 100 m/hr relative to the flow rate of entering sludge, injection means (4) for at least one flocculating agent and at least one thickener settler (6) comprising a lamellar decantation zone, a thickening zone for under-plate sludge, an extraction device for sludge and an extraction zone for clarified overflow water above the plates, the surface at the sill of said thickener being less than that obtained by dividing the flow of MIS to thicken expressed in kg MIS/day by a minimum admissible load of 200 kg/m²/day.
 13. Thickening plant according to claim 12, characterised in that it comprises injection means for a coagulating agent provided upstream of said flocculating agent injection means.
 14. Thickening plant according to either one or the other of claims 12 or 13, characterised in that the maximum total surface of said degassing zone (3) corresponds to a degassing speed of 60 m/hr relative to the flow rate of entering sludge.
 15. Thickening plant according to any one of claims 12 to 14, characterised in that it comprises at least one agitated flocculation tank, whose surface can be the whole of or part of the degassing zone.
 16. Thickening plant according to any one of claims 12 to 15, characterised in that said thickener settler (6) comprises plates (6 a) of length of between 0.5 metres and 3 metres, and typically 1.5 metres, with a spacing comprised between 5 and 15 cm, preferably between 7.5 and 10 cm, with an angle formed relative to the horizontal greater than 55°, and preferably 60°, and in which the free height between sill and bottom of the plates is comprised between 2 and 4 metres, said thickener settler having a sludge raking device (8) and comprising sensors for monitoring the height of the sludge blanket thus making it possible to maintain the latter between two pre-defined heights, each being of at least 1.5 metres.
 17. Thickening plant according to any one of claims 12 to 16, characterised in that it has no sludge recirculation loop between the lower part of said thickening zone and said flocculation zone.
 18. Thickening plant according to any one of claims. 12 to 16, characterised in that it has a recirculation loop, constituted of a piping and a sludge pump, recirculating part of the sludge between the lower part of said under-plate thickening zone and said flocculation zone. 